Neon excess in pumice: volcanological implications

We measured the noble gas abundance and Ne and Ar isotopic compositions in pumice samples from Japan and the Eolian Islands, Italy. The goal was to gi ve clues on the mechanism of air incorporation into pumice and its relation ship with eruption processes. The samples contain large excesses of atmosph eric Ne over Ar, with Ne-20/Ar-36 ratios up to 600 times the atmospheric ra tio. This excess is produced by the preferential diffusion of Ne over Ar in pumice vesicles during volcanic eruptions. The Ne-20/Ar-36 ratios measured in pumice reflect the initial quenching temperature, the cooling rate of t he eruption column and the bubble's wall thickness. The final Ne amount is likely to be proportional to the volume of preserved vesicles in pumice. Ca lculations indicate that the fraction of isolated vesicles are less than 10 % of the total, suggesting that a few amount of gas is preserved in pumice after deposition. (C) 1999 Elsevier Science B.V. All rights reserved.